This invention relates to a method for converting a feed stream of nitrogen and hydrogen to ammonia in one or more ammonia synthesis reactors located in a flow of exhaust gas from a hot gas source to provide heat transfer to the exhaust gas for pseudoisothermal operation.
Ammonia is commonly manufactured by reacting synthesis gas (syngas) components nitrogen and hydrogen in an ammonia synthesis loop including a compressor, an ammonia synthesis reactor, ammonia condensation and recovery units, and purge gas recovery. After a pass through the ammonia synthesis reactor, the unreacted synthesis gas components are typically recovered and recycled to the compressor and the reactor in a loop. Make-up synthesis gas is continuously added to the ammonia synthesis loop to provide fresh hydrogen and nitrogen.
Synthesis gas typically contains inert components introduced with the make-up syngas, including argon, methane, carbon dioxide, and others, that do not contribute to ammonia production and undesirably accumulate in the loop. Therefore, a purge gas stream is usually taken from the ammonia synthesis loop to avoid an excessive concentration of the inerts in the loop. The purge stream is typically processed in a hydrogen recovery unit, yielding a waste gas stream and a hydrogen-enriched stream for recycle to the ammonia synthesis loop. The waste gas stream comprises principally nitrogen with minor amounts of carbon dioxide, methane, hydrogen, and argon. In some cases, the waste gas can be used as a low heating value fuel gas.
A significant technological advance in the manufacture of ammonia has been the use of highly active synthesis catalysts comprising a platinum group metal such as ruthenium on a graphite-containing support, as described in U.S. Pat. Nos. 4,055,628, 4,122,040 and 4,163,775. Also, reactors have been designed to use this more active catalyst, such as a catalytic reactor bed disclosed in U.S. Pat. No. 5,250,270. Other ammonia synthesis reactors include those disclosed in U.S. Pat. Nos. 4,230,669, 4,696,799, and 4,735,780.
Ammonia synthesis schemes have also been developed based on the highly active synthesis catalyst. U.S. Pat. No. 4,568,530 discloses reacting a stoichiometrically hydrogen-lean synthesis gas in an ammonia synthesis reactor containing a highly active catalyst in the synthesis loop.
U.S. Pat. No. 4,568,532 discloses an ammonia synthesis reactor, based on a highly active catalyst, installed in series in the ammonia synthesis loop downstream from a reactor containing a conventional iron-based synthesis catalyst.
U.S. Pat. No. 4,568,531 discloses introducing a purge stream from a primary synthesis loop into a second synthesis loop using a more active synthesis catalyst to produce additional ammonia from the purge stream. Another purge stream, significantly reduced in size, is taken from the second synthesis loop to avoid an excessive buildup of inerts. The second synthesis loop, like the primary ammonia synthesis loop, employs a recycle compressor to recycle synthesis gas to the active catalyst reactors in the second synthesis loop.
U.S. Pat. No. 6,171,570 discloses converting hydrogen and nitrogen into additional ammonia from a purge stream from an ammonia synthesis loop, using an ammonia synthesis reactor that does not require staged cooling. In particular, ammonia synthesis loop purge gas is provided to an inlet of a shell and tube reactor having an ammonia synthesis catalyst on the tube-side, while boiler feedwater (BFW) is supplied to the shell-side of the reactor to provide cooling and/or to generate steam.
U.S. Patent Application Publication 20030027096, Barnett et al., now U.S. Pat. No. 6,818,028, describes a method to increase reforming furnace efficiency by preheating a reagent stream and generating synthesis gas in catalytic reactors heated in radiant, transition, and convective sections of a steam-methane reforming furnace.
Patents and publications referred to herein are hereby incorporated by reference in their entireties.